In recent years there has been great interest in the conversion of biomass to ethanol since ethanol is valuable both as a chemical feedstock and as fuel. Although ethanol can be produced from such biomass materials as molasses and corn, potentially the most attractive process for producing ethanol from biomass uses as its starting material lignocellulose products, such as wood waste products, which are much cheaper than the other biomass starting materials and are available in extremely large quantities. It is known that lignocellulosic materials can be converted to ethanol by a two-step process comprising an acid or enzymatic prehydrolysis of the lignocellulosic materials to a mixture of sugars, the most important of which is D-xylose, followed by fermentation of the xylose to ethanol using any one of a number of yeasts known to be capable of effecting this transformation.
Known prior art in this field includes U.S. Pat. No. 4,260,685 issued Apr. 7, 1981 to Pilipski. This patent discloses a method for effecting saccharification of raw cellulosic material using anhydrous liquid hydrogen chloride to yield usable glucose and other products, which in the case of many cellulosic materials comprising glucose and xylose will include D-xylose. The patent states that its products are intended for enzymatic degradation to produce alcohol.
U.S. Pat. No. 4,359,534 issued Nov. 16, 1982 to Kurtzman et al, and assigned to the same assignee as this application, describes a process for fermenting D-xylose to ethanol using the yeast Pachysolen tannophilus. The substrate used in this fermentation may be a mixture of D-xylose and glucose.
U.S. Pat. No. 4,368,268 issued Jan. 11, 1983 to Gong describes a process for the direct fermentation of D-xylose to ethanol using yeast mutants of various Candida species, including C. XF-217. The process can be either aerobic or anaerobic, and the substrate may comprise a mixture of D-glucose and D-xylose.
Besides the yeasts mentioned in the above patents, several other yeasts have been shown in recent years to be capable of converting D-xylose to ethanol. Except for Pachysolen tannophilus, these yeasts only effect fermentation of D-xylose to ethanol under aerobic conditions, and even in the case of P. tannophilus the fermentation proceeds faster aerobically. However, in none of the known processes does the yield of ethanol approach the theoretical limit of 0.51 parts by weight of ethanol per part by weight of xylose starting material. It is believed (although this invention is in no way limited by this belief) that the low yield of ethanol is largely due to oxidation of ethanol by respiration of the ethanol by the yeast. Indeed, it has been shown experimentally that when D-xylose is fermented aerobically with P. tannophilus, ethanol is formed and consumed throughout the fermentation.
It might be thought that the reduction in the yield of ethanol due to respiration by the yeast could be overcome by carrying out fermentation with P. tannophilus anaerobically, and thus in the absence of the oxygen necessary for respiration. However, not only does anaerobic fermentation with P. tannophilus suffer from the disadvantages of lack of growth and relatively slow fermentation already mentioned, but in such anaerobic fermentation the yield of ethanol decreases because of an increased production of the undesired by-product xylitol.
There is thus a need for a process for fermenting D-xylose which gives higher yields of ethanol from xylose than the prior art processes discussed above, and this invention seeks to provide such a process.